Wi-Fi Vendor Features

[中文]

ESP-WIFI-MESH

For details, see the ESP-WIFI-MESH.

Wi-Fi 80211 Packet Send

The esp_wifi_80211_tx() API can be used to:

  • Send the beacon, probe request, probe response, and action frame.

  • Send the non-QoS data frame.

It cannot be used for sending encrypted or QoS frames.

Preconditions of Using esp_wifi_80211_tx()

  • The Wi-Fi mode is station, or AP, or station/AP.

  • Either esp_wifi_set_promiscuous(true), or esp_wifi_start(), or both of these APIs return ESP_OK. This is because Wi-Fi hardware must be initialized before esp_wifi_80211_tx() is called. In ESP32, both esp_wifi_set_promiscuous(true) and esp_wifi_start() can trigger the initialization of Wi-Fi hardware.

  • The parameters of esp_wifi_80211_tx() are hereby correctly provided.

Data Rate

Side-Effects to Avoid in Different Scenarios

Theoretically, if the side-effects the API imposes on the Wi-Fi driver or other stations/APs are not considered, a raw 802.11 packet can be sent over the air with any destination MAC, any source MAC, any BSSID, or any other types of packet. However, robust or useful applications should avoid such side-effects. The table below provides some tips and recommendations on how to avoid the side-effects of esp_wifi_80211_tx() in different scenarios.

Scenario

Description

No Wi-Fi connection

In this scenario, no Wi-Fi connection is set up, so there are no side-effects on the Wi-Fi driver. If en_sys_seq==true, the Wi-Fi driver is responsible for the sequence control. If en_sys_seq==false, the application needs to ensure that the buffer has the correct sequence.

Theoretically, the MAC address can be any address. However, this may impact other stations/APs with the same MAC/BSSID.

Side-effect example#1 The application calls esp_wifi_80211_tx() to send a beacon with BSSID == mac_x in AP mode, but the mac_x is not the MAC of the AP interface. Moreover, there is another AP, e.g., “other-AP”, whose BSSID is mac_x. If this happens, an “unexpected behavior” may occur, because the stations which connect to the “other-AP” cannot figure out whether the beacon is from the “other-AP” or the esp_wifi_80211_tx().

To avoid the above-mentioned side-effects, it is recommended that:

  • If esp_wifi_80211_tx() is called in station mode, the first MAC should be a multicast MAC or the exact target-device’s MAC, while the second MAC should be that of the station interface.

  • If esp_wifi_80211_tx() is called in AP mode, the first MAC should be a multicast MAC or the exact target-device’s MAC, while the second MAC should be that of the AP interface.

The recommendations above are only for avoiding side-effects and can be ignored when there are good reasons.

Have Wi-Fi connection

When the Wi-Fi connection is already set up, and the sequence is controlled by the application, the latter may impact the sequence control of the Wi-Fi connection as a whole. So, the en_sys_seq need to be true, otherwise ESP_ERR_INVALID_ARG is returned.

The MAC-address recommendations in the “No Wi-Fi connection” scenario also apply to this scenario.

If the Wi-Fi mode is station mode, the MAC address1 is the MAC of AP to which the station is connected, and the MAC address2 is the MAC of station interface, it is said that the packet is sent from the station to AP. Otherwise, if the Wi-Fi is in AP mode, the MAC address1 is the MAC of the station that connects to this AP, and the MAC address2 is the MAC of AP interface, it is said that the packet is sent from the AP to station. To avoid conflicting with Wi-Fi connections, the following checks are applied:

  • If the packet type is data and is sent from the station to AP, the ToDS bit in IEEE 80211 frame control should be 1 and the FromDS bit should be 0. Otherwise, the packet will be discarded by Wi-Fi driver.

  • If the packet type is data and is sent from the AP to station, the ToDS bit in IEEE 80211 frame control should be 0 and the FromDS bit should be 1. Otherwise, the packet will be discarded by Wi-Fi driver.

  • If the packet is sent from station to AP or from AP to station, the Power Management, More Data, and Re-Transmission bits should be 0. Otherwise, the packet will be discarded by Wi-Fi driver.

ESP_ERR_INVALID_ARG is returned if any check fails.

Wi-Fi Vendor IE Configuration

By default, all Wi-Fi management frames are processed by the Wi-Fi driver, and the application can ignore them. However, some applications may have to handle the beacon, probe request, probe response, and other management frames. For example, if you insert some vendor-specific IE into the management frames, it is only the management frames which contain this vendor-specific IE that will be processed. In ESP32, esp_wifi_set_vendor_ie() and esp_wifi_set_vendor_ie_cb() are responsible for this kind of tasks.

Wi-Fi Channel State Information

Channel state information (CSI) refers to the channel information of a Wi-Fi connection. In ESP32, this information consists of channel frequency responses of sub-carriers and is estimated when packets are received from the transmitter. Each channel frequency response of sub-carrier is recorded by two bytes of signed characters. The first one is imaginary part and the second one is real part. There are up to three fields of channel frequency responses according to the type of received packet. They are legacy long training field (LLTF), high throughput LTF (HT-LTF), and space time block code HT-LTF (STBC-HT-LTF). For different types of packets which are received on channels with different state, the sub-carrier index and total bytes of signed characters of CSI are shown in the following table.

channel

secondary channel

none

below

above

packet

information

signal mode

non HT

HT

non HT

HT

non HT

HT

channel bandwidth

20 MHz

20 MHz

20 MHz

20 MHz

40 MHz

20 MHz

20 MHz

40 MHz

STBC

non STBC

non STBC

STBC

non STBC

non STBC

STBC

non STBC

STBC

non STBC

non STBC

STBC

non STBC

STBC

sub-carrier

index

LLTF

0~31, -32~-1

0~31, -32~-1

0~31, -32~-1

0~63

0~63

0~63

0~63

0~63

-64~-1

-64~-1

-64~-1

-64~-1

-64~-1

HT-LTF

0~31, -32~-1

0~31, -32~-1

0~63

0~62

0~63, -64~-1

0~60, -60~-1

-64~-1

-62~-1

0~63, -64~-1

0~60, -60~-1

STBC-HT-LTF

0~31, -32~-1

0~62

0~60, -60~-1

-62~-1

0~60, -60~-1

total bytes

128

256

384

128

256

380

384

612

128

256

376

384

612

All of the information in the table can be found in the structure wifi_csi_info_t.

  • Secondary channel refers to secondary_channel field of rx_ctrl field.

  • Signal mode of packet refers to sig_mode field of rx_ctrl field.

  • Channel bandwidth refers to cwb field of rx_ctrl field.

  • STBC refers to stbc field of rx_ctrl field.

  • Total bytes refers to len field.

  • The CSI data corresponding to each Long Training Field (LTF) type is stored in a buffer starting from the buf field. Each item is stored as two bytes: imaginary part followed by real part. The order of each item is the same as the sub-carrier in the table. The order of LTF is: LLTF, HT-LTF, STBC-HT-LTF. However, all 3 LTFs may not be present, depending on the channel and packet information (see above).

  • If first_word_invalid field of wifi_csi_info_t is true, it means that the first four bytes of CSI data is invalid due to a hardware limitation in ESP32.

  • More information like RSSI, noise floor of RF, receiving time and antenna is in the rx_ctrl field.

When imaginary part and real part data of sub-carrier are used, please refer to the table below.

PHY standard

Sub-carrier range

Pilot sub-carrier

Sub-carrier (total/data)

802.11a/g

-26 to +26

-21, -7, +7, +21

52 total, 48 usable

802.11n, 20 MHz

-28 to +28

-21, -7, +7, +21

56 total, 52 usable

802.11n, 40 MHz

-57 to +57

-53, -25, -11, +11, +25, +53

114 total, 108 usable

Note

  • For STBC packet, CSI is provided for every space-time stream without CSD (cyclic shift delay). As each cyclic shift on the additional chains shall be -200 ns, only the CSD angle of first space-time stream is recorded in sub-carrier 0 of HT-LTF and STBC-HT-LTF for there is no channel frequency response in sub-carrier 0. CSD[10:0] is 11 bits, ranging from -pi to pi.

  • If LLTF, HT-LTF, or STBC-HT-LTF is not enabled by calling API esp_wifi_set_csi_config(), the total bytes of CSI data will be fewer than that in the table. For example, if LLTF and HT-LTF is not enabled and STBC-HT-LTF is enabled, when a packet is received with the condition above/HT/40MHz/STBC, the total bytes of CSI data is 244 ((61 + 60) * 2 + 2 = 244. The result is aligned to four bytes, and the last two bytes are invalid).

Wi-Fi Channel State Information Configure

To use Wi-Fi CSI, the following steps need to be done.

The CSI receiving callback function runs from Wi-Fi task. So, do not do lengthy operations in the callback function. Instead, post necessary data to a queue and handle it from a lower priority task. Because station does not receive any packet when it is disconnected and only receives packets from AP when it is connected, it is suggested to enable sniffer mode to receive more CSI data by calling esp_wifi_set_promiscuous().

Wi-Fi Multiple Antennas

Please refer to the PHY.

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